The rapid increase in computing power has led to distributed computing in which many small computers are spread out among many different people. In this environment, the smaller computers need to communicate with each other and with distant and more powerful larger computers. This type of computing environment is efficient when communication is very fast. This is because each individual computer generates large amounts of data to be transferred between computers and the communications must be very fast so that individual computers will not have to wait for a long time in order to get the data that they require. Optical communication technology is very attractive for this type of application because it transmits large amounts of data very fast. In operation, a first computer sends electrical signals to an optical data link. The optical data link transforms the electrical signals into light signals and transmits the light signals to a second data link over an optical transmission media such as a fiber optic cable. The second data link transforms the light signals to electrical signals and transmits the electrical signals to a second computer.
FIG. 1 illustrates a typical prior art electrical circuit which implements an optical receiver associated with the second optical data link described above. The optical receiver includes an operational amplifier 10, a resistor 20, and a detector 40. Detector 40 has an inherent detector capacitance 45. The connection of the detector 40 to the operational amplifier 10 gives rise to a parasitic capacitance 46. The detector 40 is connected between operational amplifier 10 and bias supply 47. An optical fiber 50 is the light transmission guide which transmits light 15 to the detector 40. The detector 40 converts an optical signal into an electrical current. The electrical current is sent to node 43 which connects the operational amplifier 10 and resistor 20. The combination of the amplifier 10 and the resistor 20 amplifies the electrical signal from the detector and transforms it into a voltage, output at node 52, which can be used by subsequent electrical circuits which decode the information from the electrical signal. The amplitude of an electrical signal generated by the detector compared to the electrical noise generated at node 43 from the amplifier 10 and resistor 20 is commonly referred to as the signal to noise ratio (SNR). The SNR is proportional to the inverse of the square root of the capacitance associated with the detector 40. The capacitance associated with detector 40 includes capacitor 45 and parasitic capacitance 46. In order to obtain a high signal to noise ratio, the capacitance of the detector 45 and the parasitic capacitance 46 must be small. This typically means that the area of the detector 40 must be small to reduce the inherent capacitance of the detector 45. Also, the packaging of the detector 40 must have very little parasitic capacitance which means special packaging must be used to reduce capacitor 46. The noise sensed by the amplifier 10 at node 43 can also be reduced by physically placing the amplifier 10 close to detector 40. Further, the strength of the signal received by detector 40 is increased by transmitting a maximum amount of light to the detector 40 from the optical fiber 50. Even though the amplifier 10 is close to detector 40, the position of the amplifier does not affect the alignment requirements of the optical fiber to the detector 40.
The problem with optical communications in a distributed computing environment is that there are many connections between computers. This is a problem because in order to convert light signals into electrical signals, there must be precise alignment between the transmission media and the light to electrical signal conversion device in the optical data link. In particular, in order for the photodetector to respond fast enough to the data transmitted by the light transmission media, the photodetector must have very little capacitance. Reducing the capacitance of the photodetector requires special packaging which is expensive. Reducing the capacitance of the photodetector also requires a reduction in the area of the photodetector. Reducing the area of the photodetector requires that the alignment between the photodetector and the optical fiber must be very precise. When the alignment is very precise, the interconnections between computers become very expensive. Further, when there are many computers, this expensive connection is multiplied many times. Therefore, the optical communications between computers in a distributed computing environment is very expensive due to the packaging costs and the alignment between the light transmission media and the photodetector.
The prior art has addressed this problem in a variety of ways. A first method is to use adjustable connectors and to adjust the position of the photodetector until the response of the photodetector matches a test light signal. This method provides for low cost connectors but the cost of adjusting the cables everytime a change is made is still very expensive. Additionally, due to the numerical aperture (typically 0.2 to 0.4) of the fiber optic cables, the optical coupling efficiency is very poor which results in poor detector sensitivity. Focusing lenses are commonly used to improve this optical coupling, but the alignment of the lens to the detector is labor intensive and expensive. Another method of improving the effective alignment between the light guide and the photodetector is to merely improve the light sensitivity of the photodetector. The more sensitive the photodetector is, the more misalignment the detector can stand before it is inoperable. This is an improvement, but again, it is not very effective because photodetectors are very light sensitive to begin with. As a result, marginally improving their effectiveness does not give a large enough improvement in alignment. Moreover, improving the response in photodetectors generally involves expensive process steps such as depositing an anti-reflective coating in the light entry area of the photodetector. In either case, the expense of precise alignment is not effectively reduced.